Broadband traveling-wave amplifier



Dec. 6, 1960 'r. E. EVERHART BROADBAND TRAVELING-WAVE AMPLIFIER Filed July 22, 1955 2 Sheets-Sheet 1 1 ii 1|||l w k lllllllw r Ava ma Ira/44: Aim/aw:

"11110011110 run 1.710411111111111 lira/#1)! Dec. 6, 1960 'r. E. EVERHART 2,963,615 BROADBAND TRAVELING-WAVE AMPLIFIER Filed July 22, 1955 2 Sheets-Sheet 2 .Z Iza- Zia-6' firm/ma 72 0/1011 :TiVA'IA IIZ WWW United States Patent C BROADBAND TRAVELING-WAVE AMPLIFIER Thomas E. Everhart, Santa Monica, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed July 22, 1955, Ser. No. 523,725

2 Claims. (Cl. 315-3 This invention relates generally to traveling-wave tubes, and more particularly to an improved dispersive travelingwave amplifier having a broadband frequency response.

In a traveling-wave tube, a slow-wave structure such as a helix is capable of propagating electromagnetic energy in several different modes. One of these modes represents the fundamental wave which has a phase and a group velocity in the forward direction. In an ordinary forward wave traveling-wave tube amplifier, an electron stream is directed along the slow-wave structure at a velocity slightly greater than the phase velocity of the fundamental wave in a manner to eifect an interchange of energy from the electron stream to the wave to cause amplification of the wave. The frequency response of such an amplifier is quite broad. However, when it is desired to amplify a wave representing a mode of propagation whose phase velocity is in a direction opposite to that of the fundamental mode, i.e., a backward wave, the frequency response is very narrow band. This is because for a given voltage or velocity of the electron stream, there is only a very narrow band of frequencies in the backward wave which the electron stream will amplify or deliver energy to.

Heretofore broadening the frequency band was accomplished by having two or more helices or other anodes at different voltages which resulted in a stagger-tuned elfect in which, although the response was indeed broadened, the magnitude of amplification of the backward wave was considerably decreased and was not constant over the bandwidth.

It is, therefore, an object of the present invention to provide a broadband traveling-wave tube having a multivelocity electron beam while not suffering the disadvantages of stagger-tuned systems.

It is another object to provide in a broadband dispersive traveling-wave amplifier means for providing a multivelocity electron stream having a square-wave distribution between two limiting velocities or voltages.

It is a further object to provide in a backward-wave amplifier such a multi-velocity electron stream from the equivalent of an infinite number of cathodes each at an infinitesimally different potential and ranging between the desired upper and lower limiting potentials.

It is still a further object to provide such a cathode arrangement which is practical and simple to fabricate and utilize.

Briefly, in accordance with the present invention, these objects are achieved in the following manner. A single cathode having a shape appropriate for forming an electron beam of the desired shape or cross section is used and is directly heated in a manner such that the heating current causes a continuous potential drop across the cathode which provides a continuous electron velocity distribution between the two limits of voltage the difference between which represents the total potential drop across the cathode.

In a conventional backward-wave amplifier the desired Patented Dec. 6, 1960 beam shape is that of a hollow cylinder projected along the interior of a slow-wave structure such as a helix. In accordance with this invention, the cathode is ring shaped, the ring being severed at one point and having the desired potential difference applied to the separate ends of the ring across the gap; and this potential drop being appropriate to cause a heating current around the ring and to give the desired electron velocity distribution due to the difference in potential between successive areas of the cathode emisslve surface and the accelerating anodes.

Another embodiment of the present invention is that of a sheet beam forming cathode as used in traveling-wave tube amplifiers of the type called traveling wave magnetrons in which a sheet beam electron stream is projected between a conductive plate and a conductive periodic slow-wave structure. The cathode shape in such a traveling-wave tube is an elongated rectangle disposed with the rectangular emissive surface perpendicular to the desired beam path; and in accordance with the present invention the emlssive surface is directly heated by a potential drop along its length of a magnitude suitable to give the desired range of velocity distribution.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which an embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are In the drawings:

Fig. 1 is a' partially cut away schematic view of a typical backward wave traveling-Wave tube amplifier utilizing a cathode structure in accordance with this invention;

Fig. 2 is an enlarged perspective view of one embodiment of a cathode structure of this invention as utilized in the example of Fig. 1;

Fig. 3 is a group of graphs to aid in the description of the operation of the invention; and

Figs. 4, 5, 6 and 7 are perspective views of other embodiments of the cathode structure of this invention.

Referring now to the figures, and particularly to Fig. l, a backward-wave amplifier traveling-wave tube 10 is shown having an evacuated envelope 12 enclosing and supporting a slow-wave structure 14 and an electron gun 16. A collector electrode 18 is shown disposed at the end of glass envelope 12 opposite electron gun l6 and adapted to collect the electron beam and dissipate its kinetic energy. An input conductor 20 is coupled in a conventional manner to the collector end of slow-wave structure 14 and an output conductor 22 is coupled to the emitter end of slow-wave structure 14. Solenoid 24 encloses substantially all of the structure of tube 10 and is adapted to constrain, confine, and focus the electron beam along and within the conductive slow-wave structure 14.

Electron gun 16 comprises the annular, directly heated cathode structure 26, focusing electrode 28, and accelcrating anode 30. DC. adjustable voltage source 32 is coupled across the severed ends of cathode ring 26. Accelerating voltage source 34 is also adjustable and is coupled to the positive terminal of voltage source 32. Adjustable voltage source 36 is coupled to the accelerating anode 30 and the positive terminal of source 36 is grounded. Focusing electrode 28 is electrically coupled to cathode 26.

Referring to Fig. 2, annular cathode structure 26 is shown in more detail and a negative potential of V is shown to be that at one end of the ring, while a difierent negative potential V is shown to exist at the otherend' of the ring by virtue of voltage source 32.

Referring to Fig. 3, curve 38 shows the dispersion rela tion of a conventional backward-wave amplifier'having a unipotential cathode'which is at the potential V corresponding to an RF frequency of f,.;.

Graph 40 is a plot of the gain of such a conventional tube as a function of RF frequency; and the bandwidth for the unipotential cathode arrangement is shown to be very narrow. The gain bandpass curve shown at f -on graph 40 illustrates the manner in which the dispersion characteristic of a conventional backward-wave amplifier provides a narrow bandpass which is critically dependent upon the electron velocity of the stream.

Graph 42 shows the results of stagger-tuning an ordinary backward-wave amplifier by utilizing two or more slow-wave structures such as'helices and having them at difierent voltages which results in a broader bandwith with a lower overall gain and an irregular bandpass amplitude. The lowering of gain when the stagger-tuning is utilized is inherent in that the total beam current passing through any one helix is all at the same velocity and cannot be increased without causing backward-wave oscillation.

Graph 44 plots the dispersion characteristic of the backward-wave amplifier of Fig. 1 in which the range of velocities shown between V and V results in a frequency range shown between f and f .Graph 46 then shows the resulting bandwidth characteristic which is essentiallyrectangular with the range from f to f The maximum gain is shown to be the same 'as that according to graph .40 because in this case the total current may be considerably increased without oscillation since the current passing through the slowwave structure is made up of all. velocities in the range determined by V and V In the operation of backward-wave .amplifierttube 10, electron gun 16 emits an annular, cylindrical electron beam comprising electrons having velocities represented by the difference in potential between the particular region of cathode 26 from whence they were emitted and the potential on accelerating anode 30. Thus, it may be seen that around the periphery of the cathode 26 the elemental surface areas vary in otential in a continuous sequence. This different potential for each elemental surface area assures that the velocity of electrons emitted will likewise consist of equal sub-interval elocities which contain equal numbers of electrons, Different portions then of the electron stre m each support the amplification.

of a d fierent group of frequencies in the backward Wa"e traveling along helix 14 to thus iford a broadb nd oneration. As pointed out hereinbefore, each such Dnrt nn of the electron stream may have a curr nt iust nder t e oscillation starting beam current for the respective group of fre uencies to thus provide a h h ain ac oss the entire band idth. as shown on r ph 6 in Fig. 3.

The magnitude of the bandwidth, shown on graph 46. is determined by the frequency ditferen e between f1 and f which is in turn determined, via the dispe on ch r cteristic shown n raph 4 by the Volta e di e ence between V and V that is, the voltage magnitude of source 32. It is, therefore, seen that the bandwidth may be arbitrarily controlled by a device as simple as a ,potentiometer.

In like manner the posi ion of the bandp ss region along the frequency axis of graphs 44 and 46 may be arbitrarily controlled by the magnitude of adiustable voltage source 34 or 36; that is, by changing either or both of these voltages, the range V to V on the voltage axis of graph 44 may be shifted to tune the amplifier to the desired position along the frequency axis.

Thus there has been disclosed a dispersive travelingwave amplifier which provides maximum gain over an to a desired center frequency.

Fig. 4 shows a second embodimentofa cathode in accordance with the invention. Cathode 26 comprises a ring of n turns where n is any positive integer. Such a structure operates similarly to cathode 26 and is further adapted to mix the various electron velocities.

Fig. 5 shows a third embodiment of a cathode for a multi-velocity electron beam. Voltage source 32 is applied across a sheet beam forming cathode 27; As with cathode 2 6, the stream emitted .hasavelocity'spread along the length of the cathode byvirtue of the gradual potential drop across the cathode. Focusing. electrode 29 and accelerating anode 31 are, conventional and are adapted to produce an electron stream having the desired parameters foranyparticularapplication. Block 33 is a slow-wave circuit utilizing a multi-velocity rectangular electron beam as emitted by cathode 27.

Fig. 6 shows a second embodiment, of the cathode of Fig. 5. Voltage source 32 is applied across a rectangular cathode comprising severalsinuate turns for, analogously to cathode 26, mixing the electron velocities in the sheet beam.

Fig. 7 illustrates a further embodiment of the cathode structure of this invention. Cathode 26" is indirectly heated by filament 35 which is heated by current from adjustable voltage source 32. This embodiment is particularly adapted to provide an independence between total beam current and the magnitude of voltage source 32. In accordance with this embodiment, when the magnitude of voltage source 32 is increased to provide a greater potential drop across cathode 26", the temperature and hence emission of the cathode tends to inrease. This effect would ordinarily be undesirable, and is precluded by lowering the potential drop across heater filament 35 by decreasing the magnitude of voltage source 32 in a manner to cancel any tendency to emit a greater beam current thus providingthe desired independence between velocity spread and total beam current.

What is claimed is:

1. In a dispersive broadband backward-wave travelingwave tube for propagating electromagnetic waves along a slow-wave structure and for effecting an interchange of energy between the waves and an electron stream projected along the slow-wave structure, a cathode device adapted toemit a multi-velocity electron stream having a predetermined velocity distribution, the cathode device comprising: a cathode having twospaccd apart terminals and being adapted to be indirectly heated and to emit a predetermined total cathode beam current; a cathode heating means; and first and second variable voltage sources, said first voltage source being coupled between said terminals said cathode in a manner to provide the predetermined electron velocity distribution, said second voltage source being coupled to said heating means in a manner such that when the magnitude of voltage of said first voltage source is increased, the magnitude of voltage of said second voltage source may be concurrently decreased in a compensatory mode whereby said predetermined total cathode beam current may be lteld constant independent of the magnitude of voltage of said first voltage source.

2. In a dispersive traveling-wave tube, an electron gun of the character adapted to emit a multivelocity stream of electrons toward a slow-wave structure, the electron gun comprising: electron accelerating. means spaced along said tube and maintained at a pre-determined accelerating potentialgelectron focusing. means disposed about said tube and focusing the stream through said electron accelerating means; a variable potential voltage source for a cathode, said source providing different potentials which are negative with respect to said accelerating potential;

a'cathode having an emissive surface coupled to said voltage source, said cathode consisting of a unitary member'h-aving spaced apart terminalseach of which is coupled to a different potential of said voltage source, thus providing a continuous potential variation along said unitary cathode and said emissive surface between said cathode terminals, the potential levels of said voltage source being selected with respect to said predetermined accelerating potential so that the electrons emitted have a velocity distribution which is continuous between minimum and maximum velocities thus providing in the travelng-wave tube a frequency band of operation determined by said voltage source levels and said accelerating potential; auxiliary heating means for said cathode; and an adjustable voltage source connected to said heating means for determining the magnitude of heating effect for maintaining the total electron current in said multivelocity stream of electrons constant regardless of the magnitude of the potential difference applied to the terminals of said cathode.

References Cited in the file of this patent UNITED STATES PATENTS 2,581,243 Dodds Jan. 1, 1952 2,600,778 Klutke June 17, 1952 2,653,270 Kompfner Sept. 22, 1953 2,684,453 Hansell July 20, 1954 2,708,236 Pierce May 10, 1955 2,782,339 Nergaard Feb. 19, 1957 2,831,141 Birdsall Apr. 15, 1958 FOREIGN PATENTS 699,893 Great Britain Nov. 18, 1953 740,998 Great Britain Nov. 23, 1955 1,080,027 France May 26, 1954 OTHER REFERENCES Article by R. Kompfner, pages 1602 to 1611 Proceedings of the I.R.E. for November 1953. 

